Combustion effluents and waste products from various installations are a major source of air pollution when discharged into the atmosphere. One particularly troublesome pollutant found in many combustion effluent streams is NO.sub.2, a major irritant in smog. Furthermore, it is believed that NO.sub.2 undergoes a series of reactions known as photo-chemical smog formation, in the presence of sunlight and hydrocarbons. The major source of NO.sub.2 is NO which to a large degree is generated at such stationary installations as gas and oil-fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass furnaces, cement kilns, and oil field steam generators.
Various methods have been developed for reducing the concentration of nitrogen oxides in combustion effluents. One such method which was developed was a non-catalytic thermal deNO.sub.x method disclosed in U.S. Pat. No. 3,900,554 to Lyon which patent is incorporated herein by reference. The process disclosed in that patent teaches the reduction of NO to N.sub.2 by injecting ammonia into the combustion effluent stream at a temperature from about 975.degree. K. to about 1375.degree. K. Since the issuance of U.S. Pat. No. 3,900,554, there has been a proliferation of patents and publications relating to the injection of ammonia into combustion effluent streams for reducing the concentration of NO. It is the general consensus of the literature that ammonia injection at temperatures greater than about 1375.degree. K. would result in the generation of NO from ammonia and consequently conventional selective non-catalytic NO.sub.x reduction processes are practiced by injecting ammonia at temperatures lower than about 1375.degree. K. Because of this temperature limitation, it is difficult and sometimes not possible to apply conventional non-catalytic NO.sub.x reduction processes. This is because during the operating cycle of some boilers and heaters, the temperature range required by conventional processes corresponds to positions in the boiler or heater where it is mechanically inconvenient to inject ammonia. In at least some of these instances, this inconvenience could be overcome if the combustion effluent could be contacted with ammonia at temperatures above about 1375.degree. K. and still obtain satisfactory reductions in the concentration of NO. Furthermore, at temperatures between about 1300.degree. K. and 1375.degree. K. prior art methods were not always adequate to reduce the NO content of combustion effluent streams to environmentally desirable levels whereas the present invention provides such a method.
In addition, conventional non-catalytic thermal deNO.sub.x processes are further limited because they teach the injection of ammonia into a constant temperature, or isothermal, zone. This is limiting because in a conventional boiler or heater, operating at constant load, combustion effluents typically leave the burner flames at temperatures greater than about 1875.degree. K. As they travel through the boiler or heater they cool in stages--not continually. This staged cooling occurs because of the manner of heat removal from the combustion effluents. Heat is usually removed by heat transfer tubes which are arranged in banks with substantial cavities between the banks. Consequently, combustion effluents are rapidly cooled while they flow through a tube bank, undergo very little cooling as they pass through a cavity, rapidly cool again while passing through another tube bank, etc. U.S. Pat. No. 4,115,515 to Tenner et al, also incorporated herein by reference, teaches that the injection apparatus should be installed in a cavity in such a manner that the ammonia contacts the combustion effluent stream as the effluents come into the cavity. Such a process has the effect that the reaction time, that is the time at constant temperature during which ammonia could reduce NO.sub.x, is the total time the combustion effluents spend passing through a cavity. Unfortunately, in some boilers and heaters, this reaction time--though adequate to provide a useful NO.sub.x reduction--is not sufficient to provide as great a reduction in NO.sub.x concentration as may be environmentally desirable.
Therefore, there is still a need in the art for methods of practicing non-catalytic NO.sub.x reduction processes which will overcome, or substantially decrease, the limitations of conventional practices.